Within the European DVB (digital video broadcasting) system, digital transmission systems for satellite (DVB-S), for cable (DVB-C) and for terrestrial digital broadcasting transmission (DVB-T) have been developed and corresponding specifications have been elaborated therefor. On account of the problematic transmission conditions present on the terrestrial radio channel, the transmission method that has been prescribed in the DVB-T specification is the OFDM transmission method (orthogonal frequency division multiplexing), which can effectively combat the difficult transmission conditions.
A further important area of application for the OFDM transmission method is in high-rate wireless data transmission networks such as, for example, WLAN (wireless local area network), in particular the transmission methods defined in the standards IEEE802.11a and 11 g and also HIPERLAN/2.
The OFDM transmission method is a multicarrier transmission method in which the data stream is divided between a number of parallel (orthogonal) subcarriers that are in each case modulated with a correspondingly low data rate. As is illustrated in FIG. 1, (sub-) carrier frequencies are arranged such that they are spaced apart equidistantly from one another on the frequency scale within a transmission bandwidth K. The carrier frequencies lie on both sides of and symmetrically with respect to a center frequency fc. In the time domain, an OFDM symbol results from the superposition of all K carrier frequencies. The data transmission is effected in the form of frames or bursts, a frame containing a uniform number of OFDM symbols.
The reception and the demodulation of OFDM radio signals may be effected by conventional reception concepts based on the principle of heterodyne reception with subsequent digital quadrature mixing. However, primarily for reasons of lower power consumption and avoiding chip-external filters for image frequency suppression, preference is increasingly being given to more advanced reception concepts employing direct-mixing methods. In the case of direct-mixing receiver concepts, the radio signal that is received via an antenna and amplified is split into an inphase (I) and a quadrature (Q) branch and mixed with the output frequency of a local oscillator in both branches, the oscillator frequencies fed to the mixers being shifted reciprocally by 90° by means of a phase shifter. Consequently, the quadrature demodulation for recovering the information-carrying baseband signals is implemented using analog circuit technology in this reception concept.
Technology-dictated inaccuracies in the production process and non-idealities of the analog mixers and oscillators and also deviations between the filters in the I and Q branches give rise to so-called IQ asymmetries or IQ distortions, i.e. amplitude and phase asymmetries between the quadrature components. The real and imaginary parts of the complex baseband signal are not phase-shifted by exactly 90° relative to one another and amplitude deviations between I branch and Q branch further occur. Such IQ asymmetries may occur both in the transmitter and in the receiver. In the receiver, the IQ asymmetries in the case of OFDM based transmission systems, in the frequency domain, that is to say after the FFT transformation (Fast Fourier Transform) in the receiver, lead to a reciprocal interference between in each case two data symbols on the subcarriers whose frequencies are arranged symmetrically with respect to the center frequency fc of the OFDM spectrum (indicated hereinafter by the subcarrier indices n and −n). Each data symbol transmitted on the subcarrier n generates a signal contribution on the subcarrier with the index −n (image frequency) as a result of the IQ asymmetry added in the time domain. The superposition leads to distortions of the useful signals at the positions n and −n.
In the dissertation “Verfahren der digitalen Kompensation von Unsymmetrien der analogen Quadraturmischung in OFDM-Empfängern [Method for the digital compensation of asymmetries of the analog quadrature mixing in OFDM receivers]” by Andreas Schuchert, accepted by the faculty of electrical engineering and information technology at the Bergischen Universität-Gesamthochschule Wuppertal, chapter 4 gave a mathematical description of the IQ asymmetries and supplied a quantitative estimation of the interference contribution occurring at the image frequency of a desired signal. Chapter 6 of the aforementioned dissertation proposes two different methods for IQ error compensation by frequency domain equalization. The first method proposed therein enables a separate frequency-dependent compensation of IQ asymmetries. For the detection of the equalization parameters by means of an IQ error detector, it is also proposed to utilize the pilot carriers that are transmitted for the purpose of estimating the channel transfer function as training symbols for the purpose of estimating the IQ distortions. However, the circuit arrangements—provided for error compensation—of both methods presented have a relatively large number of function blocks and are thus characterized by a high implementation outlay.
WO 02/056523 discloses a further method by means of which transmitter- and receiver-end IQ asymmetries can be eliminated. This method is based on generating compensation signals corresponding to the IQ errors and using them for the compensation.